Others used this door to switch tunnels. *** A 2001 RAND Corporation paper written by John Arquilla and David Ronfeldt on the future of warfare used Critical Mass as a modern example of "swarming," an ancient tactic going back to Attila the Hun and his mobile horsemen/archers, who routed the conventional Roman legions again and again until the Romans finally managed to stop them at Chalons, using tactics designed to counter the swarming Huns.